Power electronics are finding increasing market space in industrial applications due to their imminent advantages in adjustable speed motor drives (ASD), unity power factor rectifications (PFC), active power filtering (APF), static var compensation (STATCOM), as well as unified power flow control (UPFC). Presently, most of their applications are in the low to medium power range from 5 kilowatts (kW) to 250 kW at the low voltage line of 208-480 volts (V), since high speed semiconductors such as insulated gate bipolar transistor (IGBT), MCT, and the like are readily available for these power and voltage levels. It is still a challenge to connect basic power converters, built from these types of semiconductor switches, directly to the medium-voltage grids (e.g., 2.3, 3.3, 4.16, 6.9 kV and the like). Solutions that allow connection to high power grids, such as silicon-carbide (SIC) switches, are still unproven and will take some time before introduction into commercial applications. Instead, research and development has focused on multilevel converters, which have emerged as a new breed of power converter options for high power applications.
Currently, the diode-clamped multilevel converter and cascaded H-bridge are the two most frequently used multilevel converter topologies. The diode-clamped multilevel converter, also called the neutral point clamped (NPC) converter, prevailed in the 1980's and found its applications in power factor correction, reactive power compensation, back-to-back intertie, adjustable speed motor drives, and unified power flow control. However, only a limited number of levels are achievable, due to the unbalanced voltage issues in the capacitors and also due to voltage clamping requirements, circuit layout, and packaging constraints.
The cascaded H-bridge has drawn considerable interest since the mid-1990s, and has been used for ASD and reactive power compensation. The modular structure provides advantages in power scalability and maintenance and fault tolerance can be achieved by bypassing the fault modules. Unfortunately, this technology requires a large number of single-phase modules accompanied by a transformer with a large number of isolated secondary windings, resulting in high manufacturing costs. Moreover, due to its single-phase nature, each converter module processes pulsating power, resulting in a high energy storage requirement, especially in low speed, constant torque applications.
Thus, it is desirable to provide low cost converters suitable for high power applications.